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Civil Engineering/Size Limit for Pad Footings

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Question
Dear Mr. Arshad,

Is there a limit to the size of a reinforced concrete pad footing?

I have a 450mm x 450mm rc column carrying quite a heavy vertical load and the pad footing size is 4.3m x 4.3m x 0.85m deep. The plan area of the pad seems out of proportion when compared with the size of the column. Do we need to apply the 45 degree vertical load distribution from the column to limit the plan size of the pad footing? Is there a limit?

Thank you for your kind attention.

Regards,

CheeVui

Answer
Hi CheeVui,

The rule of thumb I use for a pad foundation is that the ratio of length/depth should be between 3 and 4. In your case it is about 5. What this means is that your rebar will be quite heavy and you may not quite get UDL reaction below the base unless the ground is not very hard, because the pressure distribution depends on relative stiffness of base and the soil.

The pad can be larger than the 45 degree distribution- this rule only applies if you wish to make the pad mass concrete (un-reinforced).

Civil Engineering

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Arshad Khan

Expertise

I can answer any questions to do with civil and structural engineering consultancy and construction industry in East Africa and the Middle East, and specifically with the analysis and design of reinforced concrete structures. My particular expertise is in the aseismic design and optimisation of tall buildings.

Experience

Employment history: 40 years in Construction and consultancy in the UK, Kenya, Tanzania, Uganda, Ethiopia, Eritrea, South Africa, Somalia, Zambia, Austria, Bahrain, Kuwait, Doha and the U.A.E.

Organizations
.Fellow of Institution of Structural Engineers (UK) .Fellow of Institution of Civil Engineers(UK) .Member of the Institution of Engineers, Kenya .Registered Engineer, ERB, Kenya .Member of the Architectural Association of Kenya (Engineers Chapter) .Chartered Engineer (UK)

Publications
•1984: International Conference on the Art and Practice of Structural Design, London •1994: 3rd Int. Kerensky Conference in Structural Engineering, Singapore •2008: International Conference on High-Rise Towers, Abu Dhabi •2013: IEK International Conference, Kisumu, Kenya

Education/Credentials
BSc, 1st Class Hons, in Building Engineering, University of Bath, UK MSc in Concrete Structures and Technology, University of London. Diploma of Imperial College, UK.

Awards and Honors
•Science Congress Special Award (for 2-seater Hovercraft - 1968) •Institution of Civil Engineers Award for outstanding performance at Bath University (1975) •Concrete Society Postgraduate study Bursary Award (1976) •Consular Representative for British High Commission, Nairobi. (1995 to 1998) •Examiner for Institution of Civil Engineers Professional Interviews, Nairobi. (1997 to 1998) •Branch Representative in Vienna for PI assessment for Inst. of Struct. Engrs. (1999 to 2010)

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